The switched mode power supply (SMPS) is a well-known type of power converter having a diverse range of applications by virtue of its small size and weight and high efficiency, for example in personal computers and portable electronic devices such as cell phones. A SMPS achieves these advantages by switching one or more switching elements such as power MOSFETs at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted by a feedback loop (also widely referred to as a “compensation loop” or “feedback circuit”) to convert an input voltage to a desired output voltage. A SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
The feedback loop typically comprises a controller that controls the switching frequency or the switching duty cycle of the switching element(s) of the SMPS based on the output voltage and/or output current of the SMPS, in accordance with a control law defined by one or more control law parameters, to regulate the output voltage of the SMPS (in a constant voltage (CV) mode of operation) or the output (or choke) current of the SMPS (in a constant current (CC) mode of operation), keeping it in the vicinity of a predetermined value. For example, the SMPS may comprise a Proportional-Integral-Derivative (PID) controller that regulates the duty cycle (or the switching frequency, as the case may be) of the switching element(s) to keep the output voltage of the SMPS constant, in accordance with a PID control law that is characterised by the values of the P, I and D control parameters set in the PID controller.
To achieve a good feedback loop response and avoid oscillations, digital SMPS controllers operating in CV mode typically employ a filtering process that attenuates the usual voltage ripple that propagates to the feedback loop from the output of the SMPS. This voltage ripple is a substantially periodic oscillation in the output voltage of the SMPS, which is typically small in relation to the output voltage of the SMPS, and which may occur as a result of an incomplete suppression of an output alternating waveform by the SMPS's output filter. Conventional digital SMPS controllers usually attenuate the ripple component by oversampling the output voltage of the SMPS (or an error signal indicative of a difference between the output voltage and a reference voltage) and manipulating the oversampled signal using an appropriately configured Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) filter, e.g. a moving average filter or a decimation filter. Such filtering processes cause the voltage ripple component of the oversampled signal to be suppressed, thus allowing the low-bandwidth signal (i.e. voltage deviations due to transients) to be obtained for use in output voltage regulation. Where the feedback control loop of the SMPS utilises measurements of the SMPS's choke current to regulate the output current or the output voltage of the SMPS, similar techniques are used to suppress the ripple component of the choke current.